A work permit is the immigration clearance a player from outside the host country needs before they can be registered. In England the Football Association operates a points system, awarding points for international appearances, the standard of the selling league and the size of the fee and wages.
The points approach replaced a simpler rule that asked only how often a player had played for their country. Under the current system a player can qualify without international caps if they are moving from a strong league, playing regularly, and being bought and paid at a level that signals quality.
A player who falls short can still be signed through an exceptions panel, which weighs the case on its merits. That route is how clubs sign talented players from leagues that score badly in the tables, and it is discretionary rather than automatic.
The system reshaped English recruitment after freedom of movement ended, because European players who could once sign without any clearance now have to clear the same bar. It made teenage signings from Europe far harder. Young players rarely have the caps or the league minutes to score.